The present invention is a process for the production of monosilanes from the high-boiling residue resulting from the reaction of organohalides with silicon metalloid in a process typically referred to as the "direct process." The present process comprises forming a mixture comprising an organotrihalosilane and the high-boiling residue in the presence of hydrogen gas and a catalytic amount of aluminum trichloride. The present process results in consumption of the organotrihalosilane rather than a net increase which typically occurs in processes for hydrogenation of the high-boiling residue. At least a portion of the catalytic amount of aluminum trichloride may be formed in situ during conduct of the direct process and isolation of the high-boiling residue.
In the preparation of organohalosilanes by the direct process a complex mixture is formed which is typically distilled to separate monosilanes from other components present in the mixture. For example, in the "direct process," in addition to the monosilanes which in the case of the chloromonosilanes include dimethyldichlorosilane, methyltrichlorosilane, and trimethylchlorosilane there is obtained a residue which boils above the organohalosilanes, that is above about 70.degree. C. This residue is hereinafter referred to as "high-boiling residue."
The "direct process" is well described in the patent literature, for example, in Rochow, U.S. Pat. No. 2,380,995, issued Aug. 7, 1945, and Barry et al., U.S. Pat. No. 2,488,487, issued Nov. 15, 1949. The residue remaining after distillation overhead of the monosilanes is a complex mixture comprising higher boiling silicon containing compounds which have, for example, SiSi, SiOSi, and SiCSi linkages in the molecules. The residue may also contain silicon particulates and metals or compounds thereof. Typical high-boiling residues obtained from distillation of product from the direct process are described, for example, in Mohler et al., U.S. Pat. No. 2,598,435, issued May 27, 1952, and Barry et al., U.S. Pat. No. 2,681,355, issued Jun. 15, 1954.
In current commercial operations for performing the direct process, the high-boiling residue can constitute as much as five weight percent of the resultant product. Therefore, it is desirable to convert the high-boiling residue into commercially desirable products to both reduce waste disposal and to improve raw material utilization.
Wagner, U.S. Pat. No. 2,606,811, issued Aug. 12, 1952, teaches a hydrogenation process where a compound containing a halogen and the Si--Si bond is heated to a temperature of at least 300.degree. C. in the presence of hydrogen. The resultant products are monosilanes.
Atwell et al., U.S. Pat. No. 3,639,105, issued Feb. 1, 1972, describe a process where hydrosilanes are produced by contacting a disilane with hydrogen gas under pressure and heating the mixture in the presence of a transition metal catalyst such as palladium on charcoal. Atwell et al. state that the disilane may be part of a mixture from the direct process. Atwell et al. further report that when the disilane was a methylchlorosilane, the resulting product contained about four to 28 weight percent of methyltrichlorosilane. Generally, organotrihalosilanes such as methyltrichlorosilane have limited commercial usefulness and for this reason limit the usefulness of the process described by Atwell et al.
Neale, U.S. Pat. No. 4,079,071, issued Mar. 14, 1978, describes a process for preparing high yields of hydrosilanes by reacting methylchloropolysilanes with hydrogen gas under pressure at a temperature of from 25.degree. C. to about 350.degree. C. in the presence of a copper catalyst. Neale states that the methylchloropolysilanes can be those typically created as by-products of the direct process. Useful copper catalysts described by Neale include copper metal, copper salts, and complexes of copper salts with organic ligands. In some cases, Neale reports that up to 29 weight percent of methyltrichlorosilane was formed.
Ritzer et al., U.S. Pat. No. 4,393,229, issued Jul. 12, 1983, describe a process for converting alkyl-rich disilanes in a residue obtained from the manufacture of alkylhalosilanes to halogen-rich polysilanes. The process comprises treating an alkyl-rich disilane-containing residue with an alkyltrihalosilane or silicon tetrahalide in the presence of a catalyst and a catalytic amount of a hydrosilane reaction promoter at an elevated temperature. Ritzer et al. teach aluminum trichloride as a useful catalyst in their process when used with a hydrosilane promoter. Ritzer et al. further teach that the resulting halogen-rich polysilanes can, in a separate step, be cleaved to form monosilanes.
Bokerman et al., U.S. Pat. No. 5,175,329, describe a process for the production of organosilanes from the high-boiling residue resulting from the direct process that results in a net consumption of organotrichlorosilane. In the described process the high-boiling residue is contacted with an organotrichlorosilane and hydrogen gas in the presence of both a hydrogenation catalyst and a redistribution catalyst.
An object of the present invention is to provide a simple process where the high-boiling residue from a direct process for producing organohalosilanes can be converted into commercially useful monosilanes while resulting in a net consumption of organotrihalosilane. The present inventors have discovered that this objective can be met by contacting the high-boiling residue with an organotrihalosilane, hydrogen gas, and a catalytic amount of aluminum trichloride at a temperature within a range of about 150.degree. C. to 500.degree. C. Furthermore, at least a portion of the catalytic amount of aluminum trichloride can be formed in situ during conduct of the direct process and isolation of the resulting monosilanes.